Nowadays, precise determining of spatial positions is, in particular due to an increasing extent of automatisation in fabrication and processing methods, an important prerequisite for an efficient fabrication and processing of low tolerance structural members. A precise control of the beam trajectory of intense laser light is required for e.g. in laser beam material processing. Optical scanners, so-called galvanometers, are often used for this purpose. Among these are e.g. galvanometers comprising a rotatably arranged shaft to which a mirror is fixed, wherein the shaft is rotationally driven by a motor to spatially vary the beam trajectory of a laser beam impinging upon the mirror. An arrangement of two such galvanometers in series enables a two-dimensional deflection of laser light. An exact control of the beam trajectory requires a control mechanism which exactly determines the current angle of rotation of the shaft and, in case when said determined angle of rotation deviates from its set value, corrects it via a control device. The current angle of rotation is determined by means of a position detector, the output signal of which represents the control quantity.
Most optical scanners being used nowadays use so-called capacitive position detectors. In this sort of detectors the rotatably arranged shaft is coupled with an adjustable capacitor or with a dielectric arranged between the plates of the capacitor. A measurement of the capacitor's capacitance provides a quantity being connected directly to the current angle of rotation of the shaft. Capacitive position detectors as a rule provide a high angular resolution and are largely insensitive to drifting. However, the fabrication of such detectors is relatively costly and thus expensive. Further, in particular in small scanners, the detectors considerably increase the mass moment of inertia of the scanner.
For this reason, optical position detectors are increasingly used in small scanners. However, such optical position detectors had a lower angular resolution and a larger drift as compared to capacitive position detectors, so that they were used only in applications in which the demands on the exact position of the beam trajectory are not very high (e.g., laser shows). Further-developed optical position detectors achieved the required specifications in angular resolution and drift for rendering optical position detectors possible also for laser light fabrication and processing techniques that require high accuracy.
U.S. Pat. No. 5,844,673 discloses a position detector, which determines the angular position of a rotatable element, wherein the position detector comprises; a single light source directing a uniform wide angle field of light in a direction along the longitudinal axis of the rotatable element, multiple sector-shaped light detectors aligned circumferentially about the rotatable element to receive light directly from said source, and a light blocker operably connected to the rotatable element to rotate therewith about said longitudinal axis for periodically blocking portions of the light which impinge upon said light detectors from said light source. The space between the light source and the detectors is essentially free of any object, except for the light blocker, so that the light from the light source can impinge directly upon at least those portions of said detectors, which the light blocker is not blocking. I.e., the light impinges directly upon the detectors, without passing through any other object. A linear output of each of the pairs of connected detectors is utilized to provide a reliable measurement of the angular position of the rotatable element.